Default export new instance of class

Question

Will the snippets below produce new instance every time it's imported?

// 1st implementation

class ConnectionManager {
...
}

export default new ConnectionManager();

// 2nd implementation

class ConnectionManager {
...
}

const connectionManager = new ConnectionManager();
export default connectionManager;

If yes, how can I get the same instance in every import?

Answer 1

ES6 modules follow single instance pattern. That is, the instance is created when the module is loaded.

Here is an article about it.

// File: yolo.js

class Yolo {}
export let yolo = new Yolo();

// File: laser.js

import { yolo } from "./yolo.js";
// yolo is a single instance of Yolo class


// File: cat.js

import { yolo } from "./yolo.js";
// same yolo as in laster.js

Answer 2

It should be the same.

The following example uses both the implementations, imports them into 2 different files, and imports them all into single index file. Everytime an instance is created, we generate a random value for the class, and log its creation.

// ConnectionManagerImpl1.ts
class ConnectionManagerImpl1 {
    public value;

    constructor() {
        this.value = Math.random().toString(36).substring(7);
        console.log(`New ConnectionManagerImpl1 instance created: ${this.value}`)
    }
}

export default new ConnectionManagerImpl1();

---

// ConnectionManagerImpl2.ts
class ConnectionManagerImpl2 {
    public value;

    constructor() {
        this.value = Math.random().toString(36).substring(7);
        console.log(`New ConnectionManagerImpl2 instance created: ${this.value}`)
    }
}

const connectionManagerImpl2 = new ConnectionManagerImpl2();
export default connectionManagerImpl2;

---

// import1.ts
import connectionManagerImpl1 from './ConnectionManagerImpl1';
import connectionManagerImpl2 from './ConnectionManagerImpl2';

export { connectionManagerImpl1, connectionManagerImpl2 };

---

// import2.ts
import connectionManagerImpl1 from './ConnectionManagerImpl1';
import connectionManagerImpl2 from './ConnectionManagerImpl2';

export { connectionManagerImpl1, connectionManagerImpl2 };

---

// index.ts
import * as import1 from './import1';
import * as import2 from './import2';

console.log(import1)
console.log(import2)
console.log("Done")

Ran the above setup using tsc --module 'commonjs' * && node index.js

Output:

New ConnectionManagerImpl1 instance created: ddt3re
New ConnectionManagerImpl2 instance created: uv5z6
{ connectionManagerImpl1: ConnectionManagerImpl1 { value: 'ddt3re' },
  connectionManagerImpl2: ConnectionManagerImpl2 { value: 'uv5z6' } }
{ connectionManagerImpl1: ConnectionManagerImpl1 { value: 'ddt3re' },
  connectionManagerImpl2: ConnectionManagerImpl2 { value: 'uv5z6' } }
Done

As you can see, only 1 instance of ConnectionManagerImpl1 and ConnectionManagerImpl2 were created. So, both the implementation should create only 1 instance.